Maybe Clara talks him into submission?

The thing with the Master returning now is how he became redefined by the 'new' series.

The Doctor is the only Time Lord around. And so when the Master pops up, he redefines the Doctor, and their relationship is hugely different than it was before: no longer simply enemies, they are bound together as the only survivors.

So bringing him back every so often as a simple 'villain of the week' like they used to doesn't work anymore. He is more than that, bigger than that, a whole world of stories beyond simply that. Which makes him a difficult one to slot into stories and for the Doctor to just simply 'defeat' at the end of the episodes.

I think he will come back, but he has to in a big way. A REALLY big way.

And I'm sure he will come back in a big way.

The Master's the ultimate attention seeker, isn't he?

Unless

It would be cool to have the Master return as the leader of maybe even a group of timelords who managed to escape from the time lock. He may not even have been sent into the timelock - we never saw what exactly happened. He, possibly with some others, maybe got out just as the doorway closed. Though could either then go on the rampage for their own benefit, thinking themselves god-like, with the power to do what they want with the universe.
Or- maybe they aren't so bad and try to do some good in the universe, but the way they try to do that is by going to war with the Daleks, and/or the Rutans and/or Sontarans, which actually ends up causing some nasty side effects in the universe

What about a Beauty And The Beast style storyline for Clara and the Master (been watching Once Upon A Time) without the latter being redemptive?

Besides, it might interesting if the Master actually had the hots for one of the Doctor's companions.